1. Field of the Invention
The invention relates to electro-dynamic planar loudspeakers, and more particularly, to ways of controlling and/or enhancing the acoustical directivity pattern of an electro-dynamic planar loudspeaker.
2. Related Art
In the field of electro-dynamic planar loudspeakers, a diaphragm in the form of a thin film is attached in tension to a frame. An electrical circuit is applied to the surface of the diaphragm in the form of electrically conductive traces. A magnetic field is generated by a magnetic source that is mounted adjacent to the diaphragm. Typically, the magnetic source is formed from permanent magnets mounted within the frame. The diaphragm is caused to vibrate in response to an interaction between current flowing between the electrical circuit and the magnetic field generated by the magnetic source. The vibration of the diaphragm produces the sound that is generated by the electro-dynamic planar loudspeaker.
Many types of design and manufacturing challenges present themselves with regard to the manufacture of the electro-dynamic planar loudspeakers. First, the diaphragm, which is formed by a thin film, needs to be applied to the frame in tension and permanently attached thereto. Correct tension is required to optimize the resonance frequency of the diaphragm. An optimized diaphragm resonance extends the bandwidth and reduces distortion.
The diaphragm is driven by the motive force created when current passes through the conductor applied to the film within the magnetic field. The conductor on the electro-dynamic planar loudspeaker is attached directly to the diaphragm film. Accordingly, the conductor presents design challenges since it must be capable of carrying current and is preferably low in mass and securely attached to the film even at high power and high temperatures.
With the dimensional flexibility obtained with an electro-dynamic planar loudspeaker, various locations in automotive and non-automotive vehicles may be employed to house electro-dynamic planar loudspeakers. Different locations offer various advantages over other locations. The thin depth of the electro-dynamic planar loudspeaker allows it to fit where a conventional loudspeaker would not.
Other features affecting the acoustical characteristics of the electro-dynamic planar loudspeaker include the controlled directivity of the audible output from the loudspeaker. The acoustical directivity of the audible output of a loudspeaker is critical for good audio system design and performance and creates a positive acoustical interaction with the listeners in a listening environment.
The characteristic of directivity of a loudspeaker is the measure of the magnitude of the sound pressure level (“SPL”) of the audible output from the loudspeaker, in decibels (“dB”), as it varies throughout the listening environment. The SPL of the audible output of a loudspeaker can vary at any given location in the listening environment depending on the direction angle and the distance from the loudspeaker of that particular location and the frequency of the audible output from the loudspeaker. The directivity pattern of a loudspeaker may be plotted on a graph called a polar response curve. The curve is expressed in decibels at an angle of incidence with the loudspeaker, where the on-axis angle is 0 degrees.
In FIG. 8, the directivity pattern of the audible output from a loudspeaker of a given physical size is shown to vary according to the direction away from the loudspeaker and the frequency of the audible output. In the low frequency range of approximately 1 kHz, the directivity of the loudspeaker is shown to be generally omni-directional. As the frequency of the audible output from the loudspeaker increases relative to the size of the loudspeaker, the polar response curve for the loudspeaker becomes increasingly directional. The increasing directivity of the loudspeaker at higher frequencies gives rise to off-axis lobes and null areas or nodes in the polar response curves. This phenomenon is referred to as “fingering” or “lobing.”
An electro-dynamic planar loudspeaker exhibits a defined acoustical directivity pattern relative to its physical shape and the frequency of the audible output produced by the loudspeaker. Consequently, when an audio system is designed, loudspeakers possessing a desired directivity pattern over a given frequency range are selected to achieve the intended performance of the system. Different loudspeaker directivity patterns may be desirable for various loudspeaker applications. For example, for use in a consumer audio system for a home listening environment, a wide directivity may be preferred in order to cover a wide listening area. Conversely, a narrow directivity may be desirable to direct sounds such as voices, in only a predetermined direction in order to reduce room interaction caused by boundary reflections.
Often, however, space limitations in the listening environment prohibit the use of a loudspeaker in the audio system that possesses the preferred directivity pattern for the system's design. For example, the amount of space and the particular locations in a listening environment that are available for locating and/or mounting the loudspeakers of the audio system may prohibit including a particular loudspeaker that exhibits the directivity pattern intended by the system's designer. Also, due to the environment's space and location restraints, a loudspeaker may not be capable of being positioned or oriented in a manner that is consistent with the loudspeaker's directivity pattern. Consequently, the performance of the audio system in that environment cannot be achieved as intended. An example of such a listening environment is the interior passenger compartment of an automobile or other vehicle.
Because the directivity pattern of a loudspeaker generally varies with the frequency of its audible output, it is often desirable to control and/or enhance the directivity pattern of the loudspeaker to achieve a consistent directivity pattern over a wide frequency range of audible output from the loudspeaker.
Conventional direct-radiating electro-dynamic planar loudspeakers must be relatively large with respect to operating wavelength to have acceptable sensitivity, power handling, maximum sound pressure level capability and low-frequency bandwidth. Unfortunately, this large size results in a high-frequency beam width angle or coverage that may be too narrow for its intended application. The high-frequency horizontal and vertical coverage of a rectangular planar radiator is directly related to its width and height in an inverse relationship. As such, large radiator dimensions exhibit narrow high-frequency coverage and vice versa.